I see no problem if the music is out copyright or or the sheet music has been purchased as far as performance is concerned.

As regard sheet music the copyright should be expired before it is put on the internet.

During the 1939/45 war my father used to organise community singing with the lyrics
projected on a screen with a magic lanten.
That was untill there was the threat of legal action.

A bit tough don' you think?

Cousin Ken